Practical application of a fuel cell that supplies hydrogen as a fuel gas to a fuel electrode, and that supplies air as an oxidant gas to an oxidant electrode, and that generates electricity through an electrochemical reaction between hydrogen and oxygen in the air while producing water on an oxidant electrode is now being considered.
In such a fuel cell, if at the time of start of operation, the pressure of hydrogen supplied to the fuel electrode and the pressure of air supplied to the oxidant electrode are about equal to the respective pressures occurring during ordinary operation, it sometimes happens that hydrogen gas and air are unevenly distributed in the fuel electrode and the oxidant electrode, respectively, and the electrodes are degraded by electrochemical reaction caused by the uneven distribution of these gases. Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method of preventing the degradation of the electrodes of a fuel cell by causing the pressures of hydrogen and air supplied to the fuel electrode and the oxidant electrode, respectively, at the time of start of operation of the fuel cell to be higher than the ordinary supplied pressures of these gases.
However, if hydrogen gas and air are supplied at high pressure to a fuel cell when the fuel cell starts operation, it sometimes happen that the rate of rise of the voltage of the fuel cell becomes large so that the voltage of the fuel cell overshoots its upper-limit voltage. In conjunction with this problem, Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method in which when hydrogen gas and air are supplied, at the time of starting a fuel cell, at pressures that are higher than their pressures given during ordinary power generation, output electric power is extracted from the fuel cell, and is put out to a vehicle driving motor, resistors, etc., provided that the voltage of the fuel cell reaches a predetermined voltage that is lower than the upper-limit voltage.
By the way, in an electric vehicle equipped with a fuel cell, an output power command value for the fuel cell is calculated on the basis of the required electric power of a load, and the output current-voltage characteristic of a fuel cell. However, while the voltage of the fuel cell is rising from the starting voltage at the time of starting the fuel cell, the hydrogen supplied to the fuel cell and the oxygen in the air supplied thereto are consumed for raising the voltage of the fuel cell, electric current does not flow out from the fuel cell. On another hand, in a method in which after the voltage of the fuel cell, at the time of starting the fuel cell, is temporarily raised to an open-circuit voltage (hereinafter, referred to also as “OCV”), a control voltage of the fuel cell is decreased and electric power is extracted from the fuel cell, a control in which the voltage of the fuel cell is set at the OCV so that current does not flow out from the fuel cell is performed until the electricity generation of the fuel cell is permitted. In this case, however, the durability of the fuel cell is sometimes impaired.
Therefore, it is desirable that even before electricity generation of the fuel cell is permitted, the output voltage of the fuel cell be lowered from the OCV in advance in order to avoid the high-potential state. Then, if the fuel cell, at the time of start thereof, generates more electric power than required by the load, the surplus amount of electric power is charged into a secondary cell that is provided in the electric vehicle.
However, the amount of electric power with which the secondary cell is able to be further charged can sometimes be restricted depending on the state of charge of the secondary cell. Besides, the electric power storage capacity of the secondary cell itself also sometimes changes in accordance with changes in the ambient conditions of the secondary cell at the time of charging, for example, in the case of charging in a low-temperature condition (where the ambient temperature is, for example, −30° C. or lower) or in a high-temperature condition (where the temperature of the secondary cell is, for example, 50° C. or higher). Therefore, in some cases, the secondary cell can become overcharged immediately after the fuel cell is started.